Embryos of the sea urchin Paracentrotus lividus, when dissociated into single cells by exposure to Ca2 ion and Mg2 ion free seawater, reassociate spontaneously upon return to normal sea water to form aggregates capable of development to the final larval form (pluteus). The efficiency of reaggregation decreased with increasing age of the embryo. Reaggregation is prevented when a set of membrane proteins is removed from the dissociated cells by extraction with butanol. However, when the extracted proteins are added back, aggregation is restored and normal embryonic development ensues. This project proposes to purify and characterize the proteins required for restoration of embryogenesis and to determine their stage and tissue specificity.